The present invention relates to a device for applying adhesive on to sheets of packing material. In particular, the present invention relates to a gumming device for applying adhesive on to cardboard blanks on a packing machine.
Known gumming devices of the aforementioned type comprise a pair of counter-rotating rollers, respectively known as a gumming and pressure roller, arranged substantially tangent to each other along a common generating line located on the straight-line route travelled along by the blanks. The gumming roller often consists of a number of coaxial, cylindrical gumming discs spaced according to the distance between given portions of the blanks to be coated with adhesive. The said discs are partially imersed in a vessel containing adhesive, and present, on their respective cylindrical surfaces, grooves designed to feed adhesive on to the said given portions of the blanks. Fixed scraping means are provided for cleaning the cylindrical surfaces of the gumming discs, so that, subsequent to the action of the said scraping means, only the gum left inside the said grooves is transferred by the discs on to the said given portions of the blanks.
In actual practice, however, and despite careful, precise mating of the said scraping means and the said gumming roller, a thin film of adhesive inevitably clings to the entire cylindrical surface of the said gumming discs, due to the fact that a limitation is posed on the clearance between fixed scrapers and the surface of the said gumming discs, below which minimum clearance the wear on the said scrapers and gumming discs would be intolerable. Consequently, not only the said given portions of the blanks are coated by the gumming device with a thin film of gum, which fact gives rise to a number of drawbacks. One of these is that the packets produced using the said blanks, which often consist of a number of superimposed sheets of packing material, may prove faulty, due to the said sheets being stuck either together or to the contents of the packet. Another is that such fouling of the blanks also results in severe fouling of packing line components on packing machines fitted with the aforementioned type of gumming devices.